1. Field of the Invention
The present invention generally relates to a method for analyzing a sample in a liquid through using a scanning probe microscope (SPM) or a cantilever sensor.
2. Description of Related Art
SPM is a type of microscope for scanning a surface of a sample through a tiny probe and meanwhile detecting an interaction between the probe and the sample, so as to detect a shape or physical quantity of the surface of the sample, thereby generating an image. The SPM includes a scanning tunnelling microscope (STM) using a current flowing between the probe and the sample to represent the interaction there-between, or an atomic force microscope (AFM) using an atomic force between the probe and the sample to represent the interaction therebetween.
As described above, the SPM turns the interaction between the probe and the surface of the sample into an image, so as to have the following characteristics. That is, not only a sample disposed in vacuum or in the atmosphere can be observed, a sample disposed in a liquid can also be observed. To observe the sample in a liquid is implemented by using a structure, called a liquid cell, as shown in FIG. 6(a).
A sample 33 is placed in a sample holder 31, which has an opened upper surface and is capable of being fully filled up with a liquid, and a cantilever 34 with a probe 35 on a front end thereof is fixed on a base 32 and is installed on a main body of the microscope. When conducting observations in a liquid, a liquid 36 is fed into the sample holder 31 to a level that both the sample 33 and the probe 35 close to the sample 33 are immersed in the liquid 36. At this time, the liquid 36 in the sample holder 31 contacts with an external gas; as time elapses, the liquid 36 is evaporated, such that the liquid volume is reduced. During the observation, if the temperature of the liquid 36 is changed due to the evaporation, the sample 33 and the probe 35 may be thermally expanded (or shrunk); accordingly, an observation visual field may change, as the time elapses. The phenomenon is called a thermal drift, which is the reason why the distortion of the observed image occurs in SPM.
In addition, the following problems may also occur. As the temperature of the liquid change due to the evaporation of the liquid 36, the state on the surface of the sample changes accordingly, or the substance dissolved in the liquid 36 may be precipitated to contaminate the surfaces of the sample 33, the cantilever 34, and the probe 35. In addition, as for the AFM with a mechanism for detecting upper and lower displacements of the cantilever 34 by using a laser beam, as shown in FIG. 6(b), if a liquid level is lowered due to evaporation and a space is formed in an optical path of the laser, it is necessary to adjust a position of a laser source and a position of an optical detector once again. In view of the above reasons, in order to stably observe the surface of the sample for a long time, it is necessary to restrain the evaporation of the liquid in which the sample is immersed therein (hereafter referred to as “liquid for analysis”).    [Patent Document 1] Japanese Laid-Open Patent Publication NO. 2002-286614 (see [0016], FIG. 2)
As one of the solutions for preventing the evaporation of the liquid for analysis, in the conventional art, a known method using a packing to surround the sample and the liquid for analysis, so as to form a sealing space. In the above method, as shown in FIG. 7, the packing 41 formed by an O ring is clamped between the base 32, which secures s the cantilever 34, and the planar sample holder 31, such that the sample 33 and the liquid for analysis 36 are sealed within a sealing space surrounded by the sample holder 31, the base 32, and the packing 41, thereby preventing the evaporation of the liquid for analysis 36. However, when using such a method, the packing 41 is usually in the form of solid, and it is difficult to further deform after the packing 41 is collapsed to a certain extent, so it must enable a distance between the probe 35 and the surface of the sample to approach an observable distance (smaller than or equal to 1 nm) within a collapsing amount d of the packing 41. Therefore, the thickness of the sample 33 must be accurately determined with the consideration of the collapsing amount d. However, during the actual observation, it is quite difficult to accurately determine the thickness of the sample 33 for processing. Furthermore, the relative movement between the base 32 and the sample holder 31 is limited by the packing 41; thus when the observation position changes, the sample 33 or the probe 35 cannot be moved along a horizontal direction and a vertical direction to a large extent. In addition, the packing 41 usually adopts O rings made of rubber; but when an organic solvent is used as the liquid for analysis 36, the packing 41 may be dissolved by the organic solvent.
On the other hand, when it is difficult to seal the liquid for analysis, a relatively large amount of liquid for analysis is used in consideration of the evaporation. Therefore, it is possible to reduce the variation of the entire liquid for analysis with respect to the evaporated amount. However, as for the structure of the SPM, the amount of liquid for analysis that can be fed into the sample holder is limited, and when an organic solvent with a high evaporation speed is used, it is difficult to observe the sample for a long time. In addition, the following situation also exists, that is, some types of liquids for analysis may generate toxic gases during evaporation, so that the above process cannot be performed. Furthermore, if the liquid for analysis is an aqueous solution, the concentration may change due to the evaporation of the moisture.
In addition, in the method for preventing the volatilization of the liquid for analysis disclosed in the Patent Document 1, a liquid layer is disposed on the surface of the liquid used for the analysis for preventing the volatilization, wherein the liquid layer is formed by a liquid immiscible with the liquid for analysis. However, when the liquid layer for preventing the volatilization is formed on the liquid for analysis in this manner, a liquid with a specific weight smaller than that of the liquid for analysis is required to be used for forming the liquid layer. Furthermore, it is necessary to fill the liquid for analysis into the sample holder to such a level that both the sample and the probe are immersed therein, and a liquid level is formed within the sample holder; hence, a large amount of liquid for analysis is required. In addition, if a gas-liquid interface exists in the optical path of the laser for detecting the displacement of the cantilever, the sloshing of the liquid level may interfere with the optical path. Therefore, in the Patent Document 1, a method for eliminating the above problem is described, wherein a laser source and an optical detector are disposed in the liquid, or an optical fiber is disposed in the liquid for guiding the laser beam and capturing the reflected light. However, the above method has a problem that the adjustment of positions of the laser source and the optical detector, or the optical fiber is rather complicated.